An RFeB magnet, which was discovered by Sagawa (the inventor of the present invention) et al. in 1982, is characterized in that its properties are far superior to those of the previously used permanent magnets and yet it can be produced from relatively abundant, inexpensive materials, i.e. neodymium (a rare-earth element), iron and boron. Due to these merits, this magnet is currently used in various products, such as the voice coil motors for hard disk drives or similar devices, drive motors for hybrid cars or electric ears, motors for battery-assisted bicycles, industrial motors, high-quality speakers, head phones, and magnetic resonance imaging (MRI) apparatuses using permanent magnets.
The main phase of the RFeB magnet is an R2Fe14B intermetallic compound, which has a tetragonal crystal structure and possesses magnetic anisotropy (Patent Document 1). Improving the magnetic characteristics of the RFeB magnet requires making use of this magnetic anisotropy. For this reason, it is produced by a sintering process, by which a dense, uniform and fine structure can be obtained.
A sintering process is normally performed as follows: After an alloy powder of an RFeB magnet is filled into a mould, a magnetic field is applied to the alloy powder, while a pressure is applied with a pressing machine, to simultaneously perform both the molding and orientation processes. Then, the molded body is removed from the mould and heated to be sintered. Patent Document 2 discloses a different method for producing an RFeB sintered magnet; the method includes filling an alloy powder of an RFeB magnet into a filling/sintering container (filling process), orienting the alloy powder within a magnetic field without the press-molding operation (orienting process), and directly heating the powder (sintering process). By this method, an RFeB magnet having even higher magnetic characteristics can be obtained since the press-molding operation, which disorders the oriented state of the alloy powder, is omitted.
Patent Document 2 also discloses a sintered magnet production system including a closed container having an inner space maintained in an oxygen-free or inert-gas atmosphere, within which a filling means, an orienting means and a sintering means are provided, and a transfer means is also provided for transferring a filling/sintering container from the filling means to the orienting means as well as from the orienting means to the sintering means. This system can handle the alloy powder in the oxygen-free or inert-gas atmosphere throughout the entire process, thereby preventing the oxidation of the product and the deterioration of its magnetic characteristics.    Patent Document 1: Japanese Unexamined Patent Application Publication No. S59-046008    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2006-019521